Olfactory dysfunction in narcolepsy with cataplexy

Background: Narcolepsy-cataplexy (NC) is caused by the loss of hypocretin neurons. Several lines of evidence suggest a role for hypocretin in olfactory function. Recent data have documented that NC is associated with olfactory dysfunction but smell testing has been performed only in small studies. Methods: One hundred thirty NC patients were recruited from two sleep disorders centers in France and Italy. They were compared to 129 age- and gender-matched healthy controls on two well standardized olfactory tests: the Sniffin' Sticks (France) and Brief Smell Identification (Italy) tests. Olfactory dysfunction was defined as a score below the tenth centile on each smell test. Results: Altogether, olfactory deficit was higher in NC compared to controls (23.8% vs. 13.9%, p= 0.042, OR: 1.93 CI 95%, 1.01-3.66); olfactory identification deficit was found in 35.8% of NC compared to 13.9% of controls in Italy (p= 0.03), and in 11.2% vs. 8.2% in France (NS). Using the Sniffin' Sticks (France) we noted significant lower discrimination and global olfactory performance in NC compared to controls. Based on standardized criteria for Sniffin' Sticks, none of the NC patients or controls presented anosmia or severe hyposmia, but 6.4% of the NC patients had moderate hyposmia and 22.2% mild, in contrast to 6.4% of controls with only mild hyposmia. Finally except tobacco smoking, clinical potential factors including age at onset and severity of the condition did not modify olfactory performances in the whole population. Conclusion: Our study is the largest investigation of olfactory performance in NC showing that the disease per se is associated with mild/moderate dysfunction in a quarter of patients.